Fix observer: check against empty course preferences field

[TamaroWalter]

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

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📝 Description:

The course_deleted observer did not check if the courses field of the preferences is empty, which cause property_exist() to throw an error.

This is now fixed with an is_object() check. DB performance was improved by pre-filtering all records that may have the courseid in the preference field.

The unit test were improved to cover this case.

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📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.